Digital Controller for Pedestrian Crossing and Traffic Lights · 2016. 9. 22. · Digital...

Digital Controller for Pedestrian Crossing and Traffic Lights

Project Objective: -

To design and simulate, a digital controller for traffic and pedestrian lights at a pedestrian

crossing using Microsim Pspice©. The controller must be based on next-state techniques using

D-type flip-flops and multiplexers.

Specification: -

1) The lights will show red to pedestrians and green to traffic in the idle state.

2) When the pedestrian wishes to use the crossing he/she presses a button, the

traffic lights turn amber and the pedestrian lights remain red.

3) After four seconds, the traffic lights turn from amber to red, and the pedestrian

lights remain red.

4) After two seconds, the traffic lights remain red, while the pedestrian lights turn

green and a buzzer is sounded.

5) After eight seconds, the pedestrian lights turn from green to amber and to buzzer

changes pitch.

6) After four seconds the pedestrian lights change from amber to red, the buzzer

becomes silent, and at this stage, the traffic lights still remain red.

7) After a further two seconds, the system is returned to the idle state once again,

having the traffic lights green and the pedestrian lights red.

Parts used for project: -

Quite a number of parts had to be used for this project; each traffic light and pedestrian light

needs a 16 to 1 multiplexer. The counter logic needs 4 multiplexers and 4 D-type flip-flops.

The clear (or reset) function for this controller also needs a multiplexer. Due to the fact that the

multiplexers used are 16 to 1 multiplexers, every output from each multiplexer needs to go

through an inverter since 74150’s invert their output. The switching circuit also needs a D-type

flip-flop and a +5 D.C. power supply and due to the inflexibility of Pspice, two switches had to

be introduced into the switching circuit – one to turn on at a certain time and one to turn off

shortly after that interval. Below shows the response expected for the controller (P = switch) if

the switch was pressed after 4.5 seconds, the buzzers are excluded from this and will be

explained at the end of this report.

Figure 1:

Expected Responses for each Output (excluding Buzzers)

Page - 1 -

AdministratorSticky NoteInclude DIT logo, acknowledgements, content etc. A good report but a bit short.


Initial Flowchart for Pedestrian / Traffic Lights Controller

State 1 Traffic: Amber Pedestrian: Red

State 0 (Idle)

Traffic:Amber Ped: Red

State 5 Traffic: Red

Pedestrian: Red BUZZER OFF

State 4 Traffic: Red Ped: Amber

BUZZER / 1

State 3 Traffic: Red Ped: Green

BUZZER / 2


Pedestrian: Red

Switch = 1?

Page - 2 -


This flowchart is the ideal flowchart for the sequence needed, but due to the limitations

of being only allowed to use a limited amount of components a revised flowchart had to be

introduced.

A number of parameters had to be considered:

1) Only a simple clock pulse can be used, therefore more states had to be

introduced to accommodate time delays (by repeating functions for a set number

of clock pulses).

2) Another additional state had to be introduced to reset the counter logic back to 0

(and also to reset the switch to 0) after all traffic light states have been executed.

3) The simple clock pulse is set to give a positive edge triggered after every 2

seconds so as to half the number of states needed for this operation.

4) When the switch is pressed, the state one will not be activated until the next

positive edge is triggered from the clock pulse, this delay could vary from a

microsecond up to nearly two seconds.

5) The buzzer cannot be simulated in Pspice so only the input for the buzzer can be

simulated, this clearly shows when the buzzer is activated and at what level.

6) Once the switch is activated, the switch is permanently switched on until the

switch’s D-type flip-flop is reset; no matter how many times the switch is

pressed and depressed.

With all these limitations and specifications in mind the revised flowchart for this controller

was constructed.

Page - 3 -


Revised Flowchart for Pedestrian / Traffic Lights Controller

State 1-2 Traffic: Amber Pedestrian: Red

State 0 (Idle)

Traffic:Amber Ped: Red


Pedestrian: Red BUZZER OFF

Switch = 1?

State 11

Reset

State 8-9 Traffic: Red Ped: Amber

BUZZER / 1

State 4-7 Traffic: Red Ped: Green

BUZZER / 2


Pedestrian: Red

Now that the flowchart has been constructed, the truth table can be drawn up.

Page - 4 -


Present State Next State Traffic Pedestrian Clk State

Q3 Q2 Q1 Q0 D3 D2 D1 D0 G A R G A R

Swi

(P)

Reset Buzzer

0 0 0 0 0 0 0 0 0 P 1 0 0 0 0 1 1 1 0

2 1 0 0 0 1 0 0 1 0 0 1 0 0 0 1 X 1 0

4 2 0 0 1 0 0 0 1 1 0 1 0 0 0 1 X 1 0

6 3 0 0 1 1 0 1 0 0 0 0 1 0 0 1 X 1 0

8 4 0 1 0 0 0 1 0 1 0 0 1 1 0 0 X 1 ½

10 5 0 1 0 1 0 1 1 0 0 0 1 1 0 0 X 1 ½

12 6 0 1 1 0 0 1 1 1 0 0 1 1 0 0 X 1 ½

14 7 0 1 1 1 1 0 0 0 0 0 1 1 0 0 X 1 ½

6 8 1 0 0 0 1 0 0 1 0 0 1 0 1 0 X 1 1

18 9 1 0 0 1 1 0 1 0 0 0 1 0 1 0 X 1 1

20 10 1 0 1 0 1 0 1 1 0 0 1 0 0 1 X 1 1

22 11 1 0 1 1 0 0 0 0 0 0 1 0 0 1 X 0 0

24 0 0 0 0 0

Unused States:

24 12 1 1 0 0 1 1 0 1 1 0 0 0 0 1 X 0 0

26 13 1 1 0 1 1 1 1 0 1 0 0 0 0 1 X 0 0

28 14 1 1 1 0 1 1 1 1 1 0 0 0 0 1 X 0 0

30 15 1 1 1 1 0 0 0 0 1 0 0 0 0 1 X 0 0

From this truth table, maps were obtained for each counter multiplexer, each traffic light, each

pedestrian light, and for reset.

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 0 0 0

2.3 QQ (01) 0 0 1 0

2.3 QQ (11) 1 1 0 1

2.3 QQ (10) 1 1 0 1

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 0 1 0

2.3 QQ (01) 1 1 0 1

2.3 QQ (11) 1 1 0 1

2.3 QQ (10) 0 0 0 0

Page - 5 -


0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 1 0 1

2.3 QQ (01) 0 1 0 1

2.3 QQ (11) 0 1 0 1

2.3 QQ (10) 0 1 0 1

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) P 0 0 1

2.3 QQ (01) 1 0 0 1

2.3 QQ (11) 1 0 0 1

2.3 QQ (10) 1 0 0 1

Karnaugh Maps For Controller

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 0 0 0

2.3 QQ (01) 1 1 1 1

2.3 QQ (11) 0 0 0 0

2.3 QQ (10) 0 0 0 0

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 1 0 0 0

2.3 QQ (01) 0 0 0 0

2.3 QQ (11) 0 0 0 0

2.3 QQ (10) 0 0 0 0

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 1 0 1

2.3 QQ (01) 0 0 0 0

2.3 QQ (11) 0 0 0 0

2.3 QQ (10) 0 0 0 0

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 0 0 0

2.3 QQ (01) 0 0 0 0

2.3 QQ (11) 0 0 0 0

2.3 QQ (10) 1 1 0 0

Page - 6 -


0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 0 1 0

2.3 QQ (01) 1 1 1 1

2.3 QQ (11) 1 1 1 1

2.3 QQ (10) 1 1 1 1

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 1 1 1 1

2.3 QQ (01) 0 0 0 0

2.3 QQ (11) 1 1 1 1

2.3 QQ (10) 0 0 1 1

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 1 1 1 1

2.3 QQ (01) 0 0 0 0

2.3 QQ (11) 1 1 1 1

2.3 QQ (10) 1 1 0 1

Page - 7 -


Method of Transforming Karnaugh Maps into Multiplexers

Transforming the truth table to actual logic is simplified using Karnaugh Maps and by setting

each input of the multiplexer to the same value of the corresponding number on the Karnaugh

map. Also the multiplexers are set the active high. The “Q” inputs of each multiplexer

correspond to the outputs of the counter logic D-types. In this example, D3 from the counter

logic is used.

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 0 0 0

2.3 QQ (01) 0 0 1 0

2.3 QQ (11) 1 1 0 1

2.3 QQ (10) 1 1 0 1

Karnagh Map for D3

Minterm Key

Figure 2: D3 multiplexer

0.1 QQ

(00)

0.1 QQ

(01)

0.1 QQ

(11)

0.1 QQ

(10)

2.3 QQ (00) 0 1 3 2

2.3 QQ (01) 4 5 7 6

2.3 QQ (11) 12 13 13 14

2.3 QQ (10) 8 9 11 10

This method was used for each multiplexer in the controller.

Page - 8 -


Now that all the Karnaugh Maps and the truth tables are written up all that has to be done is the

simulation of the actual controller. First of all the counter logic had to be designed:

Figure 3: Counter Logic Schematic

Figure 4: Inverting Multiplexer Outputs and Reset Logic

Page - 9 -


The counter logic above uses logic hi’s and logic low’s to represent 0’s and 1’s. The

multiplexers are set to active high to give normal operation and the d-type flip-flops are pre-set

to +5 volts. The multiplexer’s outputs must be inverted since the device inverts the output.

Below shows the counter output response. Note that the counter is reset at 22 seconds. For the

moment assume that P = 1 (since we are only analysing counter logic) and that the clock is a

simple pulse. Figure 5: Counter Logic Response

Now that the counter logic is operational, the traffic light logic is constructed.

Figure 6: Traffic light logic schematic

As with before, the multiplexer output is inverted and the multiplexers are set to active high.

These multiplexers are set in accordance with the maps for each output. The value for P is set

equal to “hi” for this response since the switch is not considered yet.

Page - 10 -


Figure 7: Traffic light logic response

The traffic lights start with the initial condition of having the traffic lights green and the

pedestrian lights red, and on the first positive edge of the clock pulse state one is initiated. The

logic is initiated step by step by the counter input to each of the multiplexers (Qn).

Next the actual switch to enable to activation of the sequence is designed.

A de-bouncing switch is constructed using a d-type flip flop. This flip flop is made

“high” as soon as the switch is pressed and stays high until it is reset, no matter if the switch is

pressed repeatedly or not. The switch is simulated by 2 switches in Pspice, one switch is open

until 4.5 seconds and the other switch is closed until 4.6 seconds, therefore simulating a

switched being pressed at 4.5 seconds and being held for 0.1 second. Also included in the

diagram below is the clock pulse logic, which is fed to the counter d-type, flip-flops.

Figure 8: Switch and Clock Logic

.

The switch works well, but it should be noted that the counter logic will not change

immediately when the d-type flip flop is set “high”, but until the next positive edge from the

Page - 11 -


clock pulse is generated, this is demonstrated below. Also shown below is that the counter

logic is only activated when the switch is pressed, and is in state 0 at all other times.

Figure 9: Counter Logic Response

Finally, the buzzers have to be accounted for aswell.

Figure 10: Buzzer Logic

The power for the buzzers was simply taken from the inputs of each of the

pedestrian traffic lights that are active when the buzzers are on. For example, the buzzer is

sounded at just under 1.5 volts when the pedestrian green light is on, when the buzzer is

sounded at just over 3 volts the amber pedestrian light is on, this buzzer is pitched differently

so as to alert the pedestrians to the change in colour of lights and to urge them to get off the

street.

Page - 12 -


Figure 11: Buzzer Response

Complete Traffic / Pedestrian Light Control System

Conclusion: This controller works very well and efficiently. The only

problem from the design of the controller is that the controller doesn’t immediately start

the cycle when the switch is closed, but initiates it at the first positive edge pulse from the

clock. Apart from this discrepancy, the rest of the circuit works efficiently and safely.

The safety aspect of this controller is essential as it deals with people’s safety crossing

the road. There are eleven set states in this controller, but due to the extra possible states

provided by the multiplexers, the extra states should be defined so as to make the

pedestrians safe in the event of the controller going out of order and entering an unused

state. So in this controller the unused states are set to make all lights (pedestrian &

traffic) red, and the counter logic should return the unused state back to state 0.

How this controller could be improved.

The specification for this controller makes no restrictions on the type of

multiplexer used, but practically, for the construction of the actual controller the

multiplexer used should have as little inputs as possible so as to avoid problems with the

making of the controller. Below describes how to reduce multiplexer type from 16 to 1,

to an 8 to 1 multiplexer.

Page - 13 -


Page - 14 -

Traffic

Amber

Reduced Traffic Amber

0

1 0Q

1

0 0Q

0

0 0

0

0 0

0

0 0

0

0 0

Unused States:

0

0 0

0

0 0

The reduced multiplexer above is for the Traffic Amber Logic, this

reduction can be done for each of the traffic and pedestrian multiplexers, but not for the counter

logic which have to be left as 16 to 1 multiplexers. and 0Q 0Q are used because changes

from 0 to 1 in for the two states covered by it’s reduction and

0Q

0Q for the inverse of these values.

Also, the least significant select input ( ) had to be left out due to the halving of the possible

states for the multiplexer, but this does not affect the operation of the controller.

0Q

Project Objective: -Specification: -Parts used for project: -Karnaugh Maps For ControllerReduced Traffic Amber

Digital Controller for Pedestrian Crossing and Traffic Lights · 2016. 9. 22. · Digital...

Documents

Transcript of Digital Controller for Pedestrian Crossing and Traffic Lights · 2016. 9. 22. · Digital...